1. Field of the Invention
The present invention relates to controlled oscillators, particularly ring oscillators.
2. State of the Art
Voltage controlled ring oscillators are known. A typical example of a ring VCO (voltage controlled oscillator) is shown in FIG. 1. In its essential form, the oscillator consists merely of a number of inverting stages connected in a ring configuration. The oscillation frequency of the circuit is determined by the gate delay in the inverting stages, which can be controlled by the current through the gate, and the number of gates in the oscillator, and given by       f    osc    =      1          2      ⁢      n      ⁢              xe2x80x83            ⁢              τ        d            
where n is the number of inverters in the ring oscillator, and is necessarily odd, and xcfx84d is the gate delay per stage.
Variations on the foregoing circuit are used extensively in clock generation and clock recovery circuits, where power consumption and phase noise are not critical. (Clock generation typically requires low cycle-to-cycle jitter, as measured with an oscilloscope, but not low phase noise, as measured with a spectrum and lyzer.)
Because timing of the ring oscillator is set exclusively by active circuits, noise can be quite high. Supply voltage and process/temperature parameters greatly affect operation of the circuit and typically require additional correction circuitry. Moreover, if the signal is not full swing, a high current is needed to extract the frequency. That is, if the clock inside the ring oscillator does not have a 0 to Vdd swing because the supply of the ring is not Vdd, then the low-level signal produced by the ring must be extracted and transformed into a clean signal of 0 to Vdd swing for the rest of the circuit. Such extraction typically involves some kind of amplifier that runs at very high frequency, that consumes significant power, and that may generate substantial phase noise.
Also known are multivibrator VCOs, an example of a class of oscillators known as relaxation oscillators. A typical example of a multivibrator VCO is shown in FIG. 2. The circuit operates with transistors Q1 and Q2 alternatively turning on and off, with capacitor C being discharged through current sources I. Diodes D1 and D2 set the voltage swings at the collectors of Q1 and Q2, and transistors Q3 and Q4 act as level-shifting devices. A square-wave output appears at the collectors of Q1 and Q2, and a triangle wave appears across the capacitor, whose frequency is given by       f    o    =      1          4      ⁢              CV                              be            ⁡                          (              on              )                                )                    
The oscillation frequency is controlled by varying the current I through the current sources, and a wide frequency tuning range can be achieved. Current generation, however, can be noisy. Furthermore, because the triangle wave across the capacitor is symmetrical, the maximum obtainable frequency may be limited.
What is desired is a controlled oscillator that overcomes the foregoing disadvantages, i.e., exhibits low phase noise and allows a high oscillation frequency to be achieved. Other desirable characteristics are low power, low process variation and good supply rejection.
The present invention, generally speaking, provides a controlled oscillator that attains the foregoing objectives. The structure of the oscillator is, in general, that of a ring; however, timing of the oscillator is governed largely by an RC time constant. Since the delay is mostly RC-based, phase noise is minimal compared to an active implementation. Furthermore, in a preferred embodiment, two ring oscillators of this type are combined to form a differential oscillator circuit having still lower phase noise. In an exemplary embodiment, the ring oscillators are three-stage ring oscillators. The operation of two inverters is unaffected by the RC time constant. Because the speed of these inverters is very fast compared to the RC time constant, the oscillation frequency is quite constant versus temperature and supply voltage.